(1) Field of the Invention
The present invention relates to a viscous liquid deoxidizer, a deoxidizing sheet using the viscous liquid deoxidizer and a method for producing the deoxidizing sheet. The invention makes it possible to noticeably retard undesired oxidation reaction during production, to perform the placement of the deoxidizer in a predetermined shape on a substrate at high rate of speed by printing, painting or the like, to both easily and economically produce a thin deoxidizing sheet, and to efficiently, evenly and uniformly absorb oxygen present in the air. Since the invention relates to the thin sheet of deoxidizer, it prevents an end consumer from mistakenly swallowing the deoxidizing sheet.
(2) Description of the Related Art
When storing foods, prevention of degradation or decoloration due to oxidation reaction caused by oxygen in the air as well as of deterioration due to decomposition by aerobes is an important consideration.
In particular, when fats and oils become oxidized, it not only spoils taste and flavor of the food but may cause food poisoning due to toxicity of the oxide. The oxidation of fats and oils is promoted by oxygen, light, metal ions, heat, water content, etc. The oxidation proceeds gradually at first, but it starts proceeding rapidly after a certain period of time. Particularly, if the fats and oils include unsaturated bonds, they become oxidized to produce harmful substances such as peroxide, aldehyde, ketone, acid or the like.
In order to solve the above-mentioned problems, the following were proposed as a method for storing foods using a deoxidizer.
A first method involves production of a powdery deoxidizer by mixing a metal powder and water or the like at the optimum condition so that oxygen absorption is at the maximum level. The powdery deoxidizer thus produced is filled and sealed in a pouch made of a porous material, and the pouch containing the power deoxidizer is then placed and sealed in a non-gas-permeable container or bag together with foods (Japanese Laid-Open Patent Publication No. 62-54704).
Generally, this oxidizing pouch is produced as follows. First, the powdery deoxidizer including water content is dropped onto a predetermined region of a substrate, and then a cover made of a gas-permeable material is placed thereon. Then, the substrate and the cover are heat-sealed along the periphery so that the powdery deoxidizer is sealed inside.
The deoxidizing pouch thus produced is kept in an air-tight bag for storage or for distribution so as to prevent oxidation reaction until it is put to use.
A second method involves a deoxidizing sheet produced as follows. First, an adhesive agent is applied onto a surface of an unwoven fabric having a thickness of about 0.1 mm, and a deoxidizer having a grain size of about 100 meshes (metal powder processed by diluted hydrochloric acid) is dispersed thereon. Then, another unwoven fabric also having an adhesive agent applied on its surface is placed onto the previous unwoven fabric in such a manner that the surfaces having the adhesive agent face each other. The two unwoven fabrics are pressed so that they stick together to form a deoxidizing layer. Next, a composite film made of a nylon film (thickness: 25 .mu.m) and a polyethylene film (thickness: 40 .mu.m) is placed on the upper surface of the deoxidizing layer, and a polyethylene film provided with micro pores is placed on the bottom surface of the deoxidizing layer. Next, two of the oxidizing layers prepared as above are placed adjacent each other in such a manner that the porous polyethylene films are inside, and they are heat-sealed on three sides so as to form a pouch. Then, absorbent cotton containing water is placed in the pouch, and the opening is heat-sealed for air-tightness (Japanese Laid-Open Patent Publication No. 55-106519).
A third method involves mixing a deoxidizer having a grain size of about 1 to 50 .mu.m in a resin and then drawing the mixture of the resin so that it becomes porous (Japanese Laid-Open Patent Publication No. 2-229840).
Finally, a fourth method involves an oxygen absorbing sheet produced as follows. A resin composition made of 15 to 70% by weight of a thermoplastic resin and 30 to 85% by weight of an iron-based oxygen absorber is fabricated into an oxygen absorbing sheet having a thickness of 30 .mu.m to 5 mm, which is then drawn at least mono-directionally with a draw factor of 1.5 to 8. Then, a resin layer having an oxygen permeation rate of 10,000 cc/m.sup.2 day or more at 23.degree. C. is disposed either on one side or on both sides of this sheet (Japanese Laid-Open Patent Publication No. 5-318675).
The first method described above, where the deoxidizer takes a powder form, presents the following problems. First, the powder tends to disperse during production of the deoxidizer or the deoxidizing sheet, which degrades the work environment. Secondly, since the deoxidizer is in a powder form, it has a large surface area and its contact with the air is amply made. Therefore, oxidation reaction with the air undesirably proceeds when mixing ingredients for the deoxidizer or during the process where the deoxidizer is being placed onto the substrate. This results in both a loss of deoxidizing capability and deterioration of the quality of the deoxidizer. And, thirdly, coagulation of the oxidation product presents many difficulties such as an irregularity in drop amount of the deoxidizer onto the substrate, poor handling, complication of maintenance of production facilities, limitations imposed both on the operating hours for production facilities and the working hours for workers, and difficulties associated with the processing of the resulting coagulants.
Moreover, as described above, if the deoxidizer is in a powdery form, the undesired oxidation reaction with the air occurs during a time period between the production of the deoxidizing sheet and the packing of the deoxidizing sheet in an air-tight bag. This deteriorates the quality of the deoxidizing sheet and decreases its reliability.
In order to prevent such undesired oxidation reaction of the deoxidizer, modification can be made such that the mixing machine is structured to be air-tight and the air inside the mixing machine is replaced with nitrogen. The ingredients for the deoxidizer are then mixed uniformly. However, this makes the structure of mixing machine complex, and the mixing machine becomes expensive, resulting in high prices for the deoxidizer or deoxidizing sheet.
Moreover, since the powdery deoxidizer has poor packability, irregularities in packing amount or uneven distribution of the deoxidizer results. Therefore, the powdery deoxidizer is not suited for the production of deoxidizing sheet.
The second method is preferable in that it can uniformly distribute the deoxidizer. However, there still remain some problems as described below.
First of all, since the second method also uses the powdery deoxidizer, it has similar problems as the first method.
Moreover, since particles of the powdery deoxidizer are fixed by an adhesive agent, contact with the air is largely blocked by the adhesive agent. As a result, a desired deoxidizing effect is obtained less easily, which results in a decrease in reliability, non-uniformity in product quality, unpracticality of the product, etc. Any of these can be considered as a critical defect.
In other words, it is extremely difficult to uniformly distribute the particles of powdery deoxidizer on the surface of an adhesive agent in such a manner that the state of exposure of the particles to the air is uniform. If this is to be achieved, the production steps inevitably become complicated, thereby resulting in low productivity and extremely high production cost.
The third method described above involves mixing a deoxidizer having a particle diameter of about 1 to 50 .mu.m in a resin and then drawing the mixture of the deoxidizer and the resin so as to make the mixture porous. However, portions of the mixture which become porous by drawing are not necessarily where the deoxidizer is present. Therefore, a part of the deoxidizer is either partially or totally buried in the resin, resulting in the extremely poor contact with the air. Consequently, as in the case of the second method described above, a desired deoxidizing effect is obtained less easily, which results in a decrease in reliability, non-uniformity of product quality, unpracticality of the product, etc. Any of these can be considered as a critical defect.
The fourth method described above involves an oxygen absorbing sheet produced as follows. A resin composition made of 15 to 70% by weight of a thermoplastic resin and 30 to 85% by weight of an iron-based deoxidizer is fabricated into a sheet having a thickness of 30 .mu.m to 5 mm, which is then drawn at least mono-directionally with a draw factor of 1.5 to 8 (hereinafter referred to as sheet A). Then, a resin layer having an oxygen permeation rate of 10,000 cc/m.sup.2 day at 23.degree. C. (hereinafter referred to as layer B) is disposed either on one side or on both sides of sheet A. As in the third method, portions of the sheet which become porous by drawing are not necessarily where the deoxidizer is present. Therefore, a part of the deoxidizer is either partially or totally buried in the resin, resulting in the extremely poor contact with the air. Consequently, as in the case of the second method described above, a desired deoxidizing effect is obtained less easily, which results in a decrease in reliability, non-uniformity of product quality, unpracticality of the product, etc. Any of these can be considered as a critical defect.
In the above method, sheet A is disposed on layer B by thermal adhesion or by having a glue layer between the two. Also proposed is a method where sheet A and layer B are laminated by co-extrusion or lamination before drawing, and then the two are drawn together. However, since pores in sheet A are blocked by layer B when laminated on sheet A, the contact between the deoxidizer and the air becomes worse.